This invention relates to a process for generation of acid and for imaging, and to an imaging medium for use in this imaging process.
Some conventional non-silver halide photosensitive compositions, for example photoresists, contain molecules which are inherently photosensitive, so that absorption of a single quantum brings about decomposition of only the single molecule which absorbs the quantum. However, a dramatic increase in the sensitivity of such photosensitive compositions can be achieved if the photosensitive molecule initiates a secondary reaction which is not radiation-dependent and which effects conversion of a plurality of molecules for each quantum absorbed. For example, photoresist systems are known in which the primary photochemical reaction produces an acid, and this acid is employed to eliminate acid-labile groups in a secondary, radiation-independent reaction: the compound containing such acid-labile groups may hereinafter be referred to as a "secondary acid generator". See, for example, U.S. Pat. Nos. 3,932,514 and 3,915,706; Reichmanis et al., Chemical Amplification Mechanism for Microlithography, Chem. Mater., 3(3), 394 (1991) and Berry et al., Chemically Amplified Resists for I-line and G-line Applications, SPIE, 1262, 575 (1990). Also, U.S. Pat. No. 5,084,371 describes a radiation-sensitive mixture which contains a water-insoluble binder which comprises a mixture of phenolic and novolak polymers and which is soluble or dispersible in aqueous alkali, and an organic compound whose solubility in alkaline developer is increased by acid, and which also contains at least one acid-cleavable group, and in addition a further group which produces a strong acid upon exposure to radiation.
U.S. Pat. No. 4,916,046 describes a positive radiation-sensitive mixture using a monomeric silylenol ether, and a recording medium produced therefrom. This patent also contains an extensive discussion of radiation-sensitive compositions which form or eliminate an acid on irradiation. According to this patent, such radiation-sensitive compositions include diazonium, phosphonium, sulfonium and iodonium salts, generally employed in the form of their organic solvent-soluble salts, usually as deposition products with complex acids such as tetrafluoroboric acid, hexafluorophosphoric acid, hexafluoroantimonic acid and hexafluoroarsenic acid; halogen compounds, in particular triazine derivatives; oxazoles, oxadiazoles, thiazoles or 2-pyrones which contain trichloromethyl or tribromomethyl groups; aromatic compounds which contain ring-bound halogen, preferably bromine; a combination of a thiazole with 2-benzoylmethylenenaphthol; a mixture of a trihalomethyl compound with N-phenylacridone; .alpha.-halocarboxamides; and tribromomethyl phenyl sulfones.
A secondary acid generator needs to fulfil several differing requirements. It is desirable that the material generate a strong acid, since generation of a weak acid, such as the carboxylic acids generated by some prior art processes, may limit the types of acid-sensitive compound which can be used. The secondary acid generator is desirably of low molecular weight in order to reduce the amount of material required to generate a specific amount of acid. Finally, the secondary acid generator must be compatible with all the other components of the imaging medium in which it is to be used, and should not pose environmental problems, such as offensive smell or severe toxicity.
The aforementioned copending application Ser. No. 07/965,172 discloses that certain squaric acid derivatives are effective thermal acid generators which, upon heating, liberate squaric acid or an acid derivative thereof; thus, these squaric acid derivatives can be used in thermochemical processes for the generation of acid, and for imaging.
It has now been found that breakdown of the squaric acid derivatives described in the aforementioned copending application Ser. No. 07/965,172 can be catalyzed by acids which can protonate these derivatives, the efficiency of such protonation being dependent on the strength of the acid and thus greatest for very strong acids (superacids); the catalyzed breakdown of the squaric acid derivatives by superacids occurs rapidly at temperatures significantly lower than those required for uncatalyzed thermal breakdown of the squaric acid derivatives. Since superacid precursors are known which generate superacids on exposure to actinic (usually ultra-violet) radiation, a combination of a superacid precursor and one of the aforementioned squaric acid derivatives allows radiation-induced generation of relatively strong acid. Thus, this combination is useful for generation of acid and for imaging.